The present invention relates to seismic surveying and specifically to the detection of azimuthal variations in the earth's subsurface formations caused by crack orientation and density.
U.S. Pat. No. 3,302,164 relates to sesimic exploration for detecting fluids in formations by obtaining a ratio of the velocities of shear waves and compressional waves along a seismic line of profile. In order for the ratio to be obtained, however, the frequency spectra of the waves introduced by a seismic source had to be controlled according to the average velocity ratio expected to be encountered. An article, "Combined Use of Reflected P and SH Waves in Geothermal Reservoir Exploration," Transactions of Geothermal Resources Council, Volume 1, May 1977, discussed tests made using both compressional and shear waves in exploring for and evaluating geothermal reservoirs.
U.S. Pat. No. 4,286,332 relates to a technique of propagating seismic shear waves into the earth from compressional wave producing vibrators. U.S. Pat. No. 4,242,742 describes a technique of obtaining shear wave seismic data from surveys where impact devices for waves are used as a seismic energy source.
Recent published studies on the propagation of seismic waves useful in geophysical applications include:
S. Crampin and R. McGonigle, "The Variation of Delays in Stress-Induced Anisotropic Polarization Anomalies," Geophys. J. R. astr. Soc. (1981), v. 64, pp. 115-131.
C. Keith and S. Crampin, "Seismic Body Waves in Anisotropic Media: Reflection and Refraction at a Plane Interface," Geophys. J. R. astr. Soc. (1977), v. 49, pp. 181-208.
C. Keith and S. Crampin, "Seismic Body Waves in Anisotropic Media: Synthetic Seismograms," Geophys. J. R. astr. Soc. (1977), v. 49, pp. 225-243.
D. Booth and S. Crampin, "The Anisotropic Reflectivity Technique: Anomalous Reflected Arrivals from an Anisotropic Upper Mantle," Geophys. J. R. astr. Soc. (1983), v. 72, pp. 767-782.
D. Booth and S. Crampin, "The Anisotropic Reflectivity Technique: Theory," Geophys. J. R. astr. Soc. (1983), v. 72, pp. 755-766.
S. Crampin, "Seismic Anisotropy--a Summary," J. Geophy., v. 43 (1977), pp. 499-501.
S. Crampin, R. McGonigle and D. Bamford, "Estimating Crack Parameters from Observations of P-wave Velocity Anisotropy," Geophysics, v. 45, no. 3 (1980), pp. 345-360.
S. Crampin, "Seismic Wave Propagation Through a Cracked Solid: Polarization as a Possible Dilatency Diagnostic," Geophysics J., v. 53, no. 3 (1978), pp. 467-496.
S. Crampin, "A Review of the Effects of Anisotropic Layering on the Propagation of Seismic Waves," Geophysics J., v. 54 (1977), p. 475.
All of the above have focused on theoretical or observational investigations regarding the propagation of seismic waves in mathematical models or laboratory media having an anisotropic character with applications to the deep interior of the earth.